Creating a form to populate a table using multi-select combo box (1 Viewer)

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Hello there, I was wondering if anyone could anyone point me in the right direction for doing the following:

I have a table of groups of School numbers that belong to certain groups and I would like to create a form for users to select/deselect schools easily for each group. It is explained with screen shots in the attached word doc(within zip file).

Many thanks for any help.